This commit provides a standard interface for time intervals:
interval.floor(date)
interval.ceil(date)
interval.offset(date, k)
interval.range(start, stop, step)
All local-time intervals (such as d3.time.day) have a UTC-equivalent interval,
available as interval.utc (such as d3.time.day.utc).
Fixes#463. Note that for the time.scale, an interval function is required to
nice (such as d3.time.day). In the future, we might relax that to allow a count
of ticks, similar to the ticks method, but in that case we'd also need the
ability to floor to a given tick number (e.g., every other week) and likewise
for ceil. This seems like a reasonable first implementation.
077411a236